The carbonylation of methanol produces acetic acid:
Prior to 1970, acetic acid was made using cobalt catalysts. A rhodium carbonyl iodide catalyst was developed in 1970 by Monsanto. The rhodium catalyst is considerably more active than the cobalt catalyst, which allows lower reaction pressure and temperature. Most importantly, the rhodium catalyst gives high selectivity to acetic acid.
One problem with the original Monsanto process is that a large amount of water (about 14%) is needed to produce hydrogen in the reactor via the water-gas shift reaction (CO+H2O═CO2+H2). Water and hydrogen are needed to react with precipitated Rh(III) and inactive [RhI4(CO)2]− to regenerate the active Rh(I) catalyst. The large amount of water increases the amount of hydrogen iodide needed, which is highly corrosive and leads to engineering problems. Further, removing a large amount of water from the acetic acid product is costly.
In the late '70s, Celanese modified the Monsanto process by the adding lithium iodide salt to the carbonylation. Lithium iodide salt increases the catalyst stability by minimizing the side reactions that produce inactive Rh(III) species and therefore the amount of water needed is reduced. However, the high concentration of lithium iodide salt promotes stress crack corrosion of the reactor vessels. Furthermore, the use of iodide salts increases the iodide impurities in the acetic acid product.
In the late '90s, Lyondell Chemical Company (by its predecessors) developed a new rhodium carbonylation catalyst system that does not use iodide salt. The catalyst system uses a pentavalent Group VA oxide such as triphenylphosphine oxide as a catalyst stabilizer. The Lyondell catalyst system not only reduces the amount of water needed but also increases the carbonylation rate and acetic acid yield. See U.S. Pat. No. 5,817,869.
One challenge still facing the industry is that lowering water concentration in the methanol carbonylation results in increased formation of aldehydes and other permanganate-reducing impurities. For certain applications, acetic acid must pass permanganate time test. Therefore, efforts have been made to remove these impurities from acetic acid product.
U.S. Pat. No. 6,667,418 discloses a method for producing acetic acid by the catalytic carbonylation of methanol to obtain a reaction product stream comprising acetic acid and a minor amount of acetaldehyde. The acetaldehyde content in the reaction product stream is reduced by oxidation to convert at least a portion of the acetaldehyde in the stream to acetic acid or further to CO2 and H2O. The oxidized stream is then directed to the purification section, the reaction section, or both whereby the concentration of acetaldehyde is reduced.
U.S. Pat. No. 6,143,930 discloses a method to manufacture high purity acetic acid by reducing acetaldehyde from a light phase of an intermediate stream in the reaction process by employing a multiple distillation process coupled with an optional extraction of acetaldehyde. The distillation process involves first distilling a light phase to concentrate the acetaldehyde, and then separating it in a second distillation tower. The second distillation serves to remove acetaldehyde from methyl iodide, methyl acetate, and methanol mixture. As an optional third step, the twice distilled stream may be directed to an extractor to remove any remaining quantities of methyl iodide from the aqueous acetaldehyde stream to obtain acetic acid as a final product in greater than 99% purity.
U.S. Pat. No. 5,783,731 discloses a process to reduce carbonyl impurities in a carbonylation reaction for the production of acetic acid. The methyl iodide recycle stream, which is directed to a carbonylation reactor for carbonylating methanol or methyl acetate to acetic acid, is treated to remove aldehyde impurities by reacting the methyl iodide stream formed in the reaction with an aqueous amino compound which reacts with the aldehydes to form water soluble nitrogenous derivatives, separating an organic methyl iodide phase from an aqueous derivative phase and distilling the methyl iodide phase to remove heavier impurities.
U.S. Pat. No. 5,155,266 discloses a method for improving the permanganate time of acetic acid produced by the low water carbonylation of methanol in a reaction medium comprising methanol, carbon monoxide, from 0.5% to 30 wt % of methyl acetate, from 5% to 20 wt % of methyl iodide, from 20% to 20 wt % of soluble alkali metal iodide, and a halogen-promoted rhodium catalyst in the presence of less than 14 wt % of water. The method comprises contacting said acid with ozone in the presence of an oxidation catalyst.
All above known methods are necessarily performed in an integrated acetic acid production process. They require the use of complicated distillation equipment and oxidizing agents such as peroxides and ozone. Therefore, these methods are not suitable for the post-treatment acetic acid product which has been made.
New method for removing aldehyde and other permanganate-reducing impurities from acetic acid product is needed. Ideally, the method could be performed conveniently and effectively.